Mid-call control like "Stop my video" fails if a call occurs from a VTC endpoint in Skype for Business Server 2015

Consider the following scenario:

• You integrate Video Interop Server (VIS) with Cisco Unified Communication Manager (CUCM) in a Microsoft Skype for Business Server 2015 environment.
• You use a Cisco teleconferencing system (VTC) endpoint, such as SX20 to make a call to a Skype for Business client.
• An Audio/Video call is established between the endpoint and the client.

In this scenario, after some seconds, when the Skype for Business client makes some mid-call controls, such as clicking stop my video or start my video, or both options, the call is dropped.

This issue occurs because after Skype for Business client answers an incoming call from Cisco VTC endpoint, VIS keeps sending INVITEs request to Cisco VTC or CUCM every second. However, Cisco VTC endpoint starts to reply 503 service unavailable from the 24th INVITE onwards. Therefore, VIS drops both legs of the call when Skype for Business client does a mid-call control operation.

To fix this issue, install the December 2017 cumulative update 6.0.9319.510 for Skype for Business Server 2015, core components.